1. Field of the Invention
The present invention relates to a stencil making apparatus for thermo-sensitively perforating stencil sheet, particularly to an apparatus and a method of stencil making capable of restraining occurrence of moire on stencil sheet as perforated.
2. Description of the Related Art
In a stencil printer or the like, there is provided a stencil making apparatus for perforating stencil sheet used in printing. The stencil making apparatus perforates stencil sheet by applying heat thereto. The perforated stencil sheet is transferred to a printing section and image is formed on print sheet by supplying ink from the perforation.
The stencil sheet is grossly classified into two kinds, that is, 1) one constituted by film and gauze (supporter having mesh structure) and 2) one comprising film and Japanese paper. Particularly, the stencil sheet of 1) is frequently used in a simplified type stencil printer.
In the following, an explanation will be given of a stencil making apparatus for perforating stencil sheet by using a supporter having a mesh structure of 1).
The stencil making apparatus is provided with a thermal print head and a platen and stencil sheet is thermal-sensitively perforated by the thermal print head while feeding the stencil sheet in a direction orthogonal to the thermal print head by rotating the platen. At this occasion, in producing stencil image, main scanning is carried out in the width direction of the stencil sheet by the thermal print head and sub-scan is carried out in the length direction of the stencil sheet by rotating the platen.
FIGS. 6(a) and 6(b) are drawings showing a perforated state of stencil sheet.
As shown by FIG. 6(a), perforations K on stencil sheet S are formed by the resolution of a thermal print head (pitch P1 of arranging heat generating members). As illustrated, arrangement of the perforations K is formed in one straight line directed in a main scanning direction X of the thermal print head (direction of arranging heat generating members). Further, the perforations K are formed by a predetermined pitch P2 (P1=P2 in the illustrated example) in a sub scan direction Y for forming image.
As shown by FIG. 6(b), supporters ST of the stencil sheet S are formed in a mesh-like shape and vertically and horizontally arranged regularly at a predetermined pitch P3 in directions the same as the directions of arranging the perforations K (main scanning direction X and sub scan direction Y).
However, in the case of perforation using stencil sheet constituted by a film and the supporters having the mesh structure, depending on combinations of the resolutions P1 (P2) of the thermal print head and the mesh pitch P3 of the supporters of the stencil sheet, there is a case in which significant moire occurs on the stencil sheet as perforated. The moire occurs similarly on print sheet as printed.
For example, when the resolution P1 of the thermal print head and the pitch P2 of sub scan are 300 dpi (P1=84.67 xcexcm) and the pitch P3 of the mesh is 200 dpi (P3=127 xcexcm), the respective pitches establish a relationship of 3:2, a predetermined regularity is produced therebetween and accordingly, there occurs moire which is easily recognizable by optical observation.
Most significant moire occurs when the resolution of the thermal print head and the pitch of the mesh are provided with similar values (dpi).
Conversely, by adequately combining the resolution of the thermal print head and a mesh interval of the stencil sheet, moire can be made comparatively inconspicuous by diminishing the regularity between the pitches of both. However, in recent years, the both values (dpi) are proximate to each other by improvement of the resolution and it is difficult to dispense with occurrence of moire.
Otherwise, in order to suppress moire, there is provided a method in which a direction of arranging mesh of supporters and a main scanning direction of a thermal print head are inclined relative to each other such that the directions are not orthogonal to each other. In this case, the thermal print head must be attached to be inclined to a transfer direction of stencil sheet and therefore, structure of a stencil machine becomes complicated and the transfer becomes unstable.
Further, although it is conceivable to incline the direction of arranging the mesh of the supporters in fabricating the stencil sheet, the constitution is very inefficient in view of production and is not practical.
The present invention has been carried out in order to resolve the above-described problem and it is an object of the present invention to provide an apparatus and a method of stencil making capable of diminishing occurrence of moire by perforating stencil sheet having supporters in an arrangement having a regularity.
In order to achieve the above-described object, according to a first aspect of the present invention, there is provided a stencil making apparatus for thermal-sensitively perforating stencil sheet by a thermal print head, the stencil making apparatus comprising controlling means for driving contiguous N pieces of heat generating elements of the thermal print head while shifting heat generating timings at every 1/N of a period of one line of main scanning in perforating inputted image data of the one line of the main scanning.
Further, according to a second aspect of the present invention, there is provided a stencil making apparatus for perforating stencil sheet having supporters in a mesh structure having a regular pitch, the stencil making apparatus comprising a thermal print head arranged with a plurality of pieces of heat generating members at a regular pitch and controlling means for dividing inputted image data of one line of main scanning such that contiguous N pieces of the heat generating elements of the thermal print head generate different lines of print data of N lines divided into 1/N of the period of the one line and outputting the N lines of the print data to the thermal print head at different timings.
Further, according to a third aspect of the present invention, there can be constructed a constitution in which the controlling means outputs the N lines of the print data to the thermal print head during a time period of the period of the one line of the main scanning.
Further, according to a fourth aspect of the present invention, there may be constructed a constitution further comprising setting means for setting a value of N (at least, N is equal to or larger than 2) such that a regularity is diminished in a relationship between a pitch of supporters of the stencil sheet and a pitch of the heat generating members of the thermal print head, wherein the controlling means controls to divide the print data of the N lines based on the value of N set by the setting means and output timings thereof.
According to a fifth aspect of the present invention, there is provided a stencil making method of thermal-sensitively perforating stencil sheet by a thermal print head wherein in perforating inputted image data of one line of main scanning, contiguous N pieces of heat generating elements of the thermal print head are driven while respectively shifting heat generating timings at every 1/N of a period of the one line of the main scanning.
According to the above-described constitution, in the thermal print head, contiguous heat generating members do not generate heat simultaneously but generate heat at timings different from each other and accordingly, an angle may be constituted between the stencil sheet and a main scanning direction of the thermal print head.
Thereby, even in the case in which a regularity is present between a mesh pitch of the supporters of the stencil sheet and the resolution of the thermal print head, the regularity can be diminished and occurrence of moire on the stencil image can be diminished.